The present invention relates generally to wideband impulse radio systems. More particularly, this invention pertains to systems and methods for modulating and demodulating wideband impulse radio signals.
There is a continuing need for the development of advanced wireless devices for communications of voice and data, for materials measurement, navigation, environmental sensing, radar, security and numerous other civilian and military applications of radio technology. Improvements are needed in the underlying technology to provide greater reliability, greater accuracy, lower power consumption, lower cost, reduced size, and efficient use of the limited available spectrum. Conventional narrow band AM, FM, CDMA, TDMA and similar wireless communications methods and systems have not fully met these needs.
However, there is an emerging technology called Impulse Radio (including Impulse Radar) (xe2x80x9cIRxe2x80x9d) that offers many potential advantages in addressing these needs. Impulse radio was first fully described in a series of patents including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,303,108 (issued Nov. 8, 1994), all invented by Larry W. Fullerton and assigned to Time Domain Corporation. The disclosure of each of these patents is incorporated in this patent specification by reference.
Impulse radio systems are generally characterized by their transmission of short duration, broad band pulses on a relatively low duty cycle. In some systems these pulses may approach a Gaussian monocycle, where the instantaneous pulse bandwidth is on the order of the center frequency. The short pulse, low duty cycle mechanism produces a processing gain that may be utilized for interference rejection and channelization. Because of the extremely wide instantaneous bandwidth of the pulse, the available processing gain far exceeds what is achieved using typical conventional spread spectrum methods. This enables the utilization of many more channels at higher dynamic ranges and higher data rates than are available in the typical conventional spread spectrum system.
Impulse radio systems have further advantages in the resistance to multipath effect. Because impulse radio signals are divided in time rather than in frequency, time related effects, such as multipath interference, can be separated, resulting in lower average power and higher reliability for a given power level.
Impulse radio techniques are also useful in radar systems. Impulse radar systems enjoy the combined advantages of very short pulses at relatively low frequencies. The short pulses result in high resolution and the low frequency gives relatively high material penetration. If a radar system used a pulse of equivalent bandwidth at a higher carrier frequency, the material penetration properties would usually be impaired. This combined advantage enables IR radio to be used for ground penetrating radar for inspection of bridges, roads, runways, utilities and the like, and security applications, and to xe2x80x9cseexe2x80x9d through walls for emergency management situations.
Impulse radio systems are not limited to transmitting and receiving Gaussian monocycle pulses. However, some basic impulse radio transmitters attempt to emit short Gaussian monocycle pulses having a tightly controlled average pulse-to-pulse interval. A Gaussian monocycle is the first derivative of the Gaussian function. However, in a real world environment, a perfect Gaussian pulse is not achievable. In the frequency domain, this results in a slight reduction in the signal bandwidth. The signals transmitted by an IR transmitter, including Gaussian monocycles, signals having multiple cycles in a Gaussian envelope, and their real world variations, are sometimes called impulses. The Gaussian monocycle waveform is naturally a wide bandwidth signal, with the center frequency and the bandwidth dependent on the width of the pulse. The bandwidth is approximately 160% of the center frequency. In practice, the center frequency of a monocycle pulse is approximately the reciprocal of its length, and its bandwidth is approximately equal to 1.6 times the center frequency. However, impulse radio systems can be implemented where the transmitted and/or received signals have waveforms other than an ideal Gaussian monocycle.
Most prior art wireless communications systems use some variation of amplitude modulation (AM) or frequency modulation (FM) to communicate voice or data with a radio carrier signal. However, impulse radio systems can communicate information using a novel technique known as pulse position modulation. Pulse position modulation is a form of time modulation in which the value of each instantaneous value or sample of a modulating signal (e.g., a voice or data signal) is caused to change or modulate the position in time of a pulse. In the frequency domain, pulse position modulation distributes the energy over more frequencies.
In some impulse radio communications, the time position (pulse-to-pulse interval) is preferably varied on a pulse-by-pulse basis by two separate components: an information component and a pseudo-random code component. Prior art spread spectrum radio systems make use of pseudo-random codes to spread a narrow band information signal over a relatively wide band of frequencies. A spread spectrum receiver then correlates these signals to retrieve the original information signal. Unlike conventional spread spectrum systems, impulse radio systems achieve spreading through the spectrum of the waveform and thus do not need the pseudo-random code for energy spreading. The pseudo-random code is used to smooth the comb spectrum, reject interference, and for channelization. In some applications, impulse radio transmitters can use pulse widths of between 20 and 0.1 nanoseconds (ns) and pulse-to-pulse intervals of between 2 and 5000 ns. These narrow monocycle pulses have an inherently wide bandwidth. This bandwidth is the range of frequencies in which one or more signal characteristics fall within specified limits, such as for example, six dB below the peak spectral density.
Thus, in some impulse radio systems, the pseudo-random noise (PN) code component is used for different purposes: channelization; energy smoothing in the frequency domain; and interference resistance. Channelization is a procedure employed to divide a communications path into a number of channels. In a system that does not use a coding component, differentiating between separate transmitters would be difficult. PN codes create channels, if there is low correlation and/or interference among the codes being used. If there were a large number of uncoded impulse radio users within a confined area, there might be mutual interference. Further, while the use of the PN coding minimizes that interference, as the number of users rises the probability of an individual pulse from one user""s sequence being received simultaneously with a pulse from another user""s sequence increases. Fortunately, impulse radio systems can be designed so that they do not depend on receiving every pulse. In such systems, the impulse radio receiver can perform a correlating, synchronous receiving function (at the RF level) that uses a statistical sampling of many pulses to recover the transmitted information. Advanced impulse radio systems may utilize multiple pulses to transmit each data bit of information, and each pulse may be dithered in time to further smooth the spectrum to reduce interference and improve channelization. These systems may also include a sub-carrier for improved interference resistance and implementation advantages. In other embodiments of an impulse radio system, however, each xe2x80x9cbitxe2x80x9d of transmitted information can be represented by a single pulse, with no coding component.
Energy smoothing in the frequency domain insures that impulse radio transmissions interfere minimally with conventional radio systems. In some impulse radio systems, optimal energy smoothing is obtained by applying to each pulse a PN code component time dither having a much larger magnitude than the information component time dither.
Besides channelization and energy smoothing, the PN coding can also make impulse radio highly resistant to interference from all radio communications systems, including from other impulse radio transmitters. This is critical, as any other signals within the band occupied by an impulse signal can act as interference to the impulse radio. Because there are no unallocated bands at or above 1 GHz available for impulse radio systems, they must share spectrum with other conventional and impulse radios without being adversely affected. Using a PN code can help impulse systems discriminate between the intended impulse transmission and transmissions from others.
The impulse radio transmitter generally includes a time base, such as a voltage-controlled oscillator, that generates a periodic timing signal with sub-nanosecond timing accuracy. The periodic timing signal is supplied to a code source and a code time modulator. The code source is typically a storage device for storing pseudo-random noise (PN) codes that have low cross correlation among the codes in a set and will include means for generating a code signal representative of the code sequence. The code source monitors the periodic timing signal to permit the code signal to be synchronized to the code time modulator. The code time modulator uses the code signal to modulate the periodic timing signal for channelization and smoothing of a final emitted impulse radio signal. The output of the code time modulator is called the coded timing signal.
In many IR systems, the impulse radio receiver is a direct conversion receiver with a single conversion stage that coherently converts a series of pulses into a baseband signal. The baseband signal is the information channel for the basic impulse radio communications system. In such systems, pulse trains, not single pulses, are used for communications. Accordingly, the impulse radio transmitter in such systems generates a train of pulses for each bit of information. The data rate of such an impulse radio transmission is only a fraction of the periodic timing signal used as a time base. Each data bit modulates the time position of many of the pulses of the periodic timing signal. This yields a modulated, coded timing signal that comprises a train of identical pulses for each single data bit. Some impulse radio receivers typically integrate 200 or more pulses to yield the baseband output. Other systems use a xe2x80x9cone pulse per bitxe2x80x9d information transmission scheme. The number of pulses over which the receiver integrates is dependent on a number of variables, including pulse rate, bit rate, interference levels, and range.
An example of an ultra wideband impulse radio communication system having one subcarrier channel is shown in block diagram form in FIGS. 1a and 1b. The transmitter 602 (FIG. 1b) comprises a time base 604 that generates a periodic timing signal 606. The time base 604 typically comprises a voltage controlled oscillator (VCO), or the like, having a high timing accuracy and low jitter, on the order of picoseconds (ps). The voltage control to adjust the VCO center frequency is set at calibration to the desired center frequency used to define the transmitter""s nominal pulse repetition rate. The periodic timing signal 606 is supplied to a precision timing generator 608.
The precision timing generator 608 supplies synchronizing signals 610 to the code source 612 and utilizes the code source output signal 614 together with an internally generated subcarrier signal (which is optional) and an information signal 616 to generate a modulated, coded timing signal 618. The code source 612 comprises a storage device such as a random access memory (RAM), read only memory (ROM), or the like, for storing suitable PN codes and for outputting the PN codes as a code signal 614. Alternatively, maximum length shift registers or other computational means can be used to generate the PN codes.
An information source 620 supplies the information signal 616 to the precision timing generator 608. The information signal 616 can be any type of intelligence, including digital bits representing voice, data, imagery, or the like, analog signals, or complex signals.
A pulse generator 622 uses the modulated, coded timing signal 618 as a trigger to generate output pulses. The output pulses are sent to a transmit antenna 624 via a transmission line 626 coupled thereto. The output pulses are converted into propagating electromagnetic pulses by the transmit antenna 624. In the present embodiment, the electromagnetic pulses are called the emitted signal, and propagate to an impulse radio receiver 702, such as shown in FIG. 1a, through a propagation medium, such as air, in a radio frequency embodiment. In a preferred embodiment, the emitted signal is wide-band or ultrawide-band, approaching a monocycle pulse. However, the emitted signal can be spectrally modified by filtering of the pulses. This filtering will usually cause each monocycle pulse to have more zero crossings (more cycles) in the time domain. In this case, the impulse radio receiver can use a similar waveform as the template signal in the cross correlator for efficient conversion.
The receiver 702 (FIG. 1a) is designed for reception of digital data wherein one or more pulses are transmitted for each data bit and comprises a receive antenna 704 for receiving a propagated impulse far radio signal 706. A received signal 708 from the receive antenna 704 is coupled to a cross correlator or sampler 710 to produce, a baseband output 712. The cross correlator or sampler 710 includes multiply and integrate functions together with any necessary filters to optimize signal to noise ratio.
The receiver 702 also includes a precision timing generator 714, which receives a periodic timing signal 716 from a receiver time base 718. This time base 718 is adjustable and controllable in time, frequency, or phase, as required by a lock loop in order to lock on the received signal 708. The precision timing generator 714 provides synchronizing signals 720 to the code source 722 and receives a code control signal 724 from the code source 722. The precision timing generator 714 utilizes the periodic timing signal 716 and code control signal 724 to produce a coded timing signal 726. The template generator 728 is triggered by this coded timing signal 726 and produces a train of template signal pulses 730 ideally having waveforms substantially equivalent to each pulse of the received signal 708. The code for receiving a given signal is the same code utilized by the originating transmitter 602 to generate the propagated signal 706. Thus, the timing of the template pulse train 730 matches the timing of the received signal pulse train 708, allowing the received signal 708 to be synchronously sampled in the correlator 710. The correlator 710 ideally comprises a multiplier followed by a short term integrator to sum the multiplier product over the pulse interval. Further examples and details of correlation and sampling processes can be found in commonly owned U.S. Pat. Nos. 4,642,317, 4,813,057, and 4,979,186, which are incorporated herein by reference, and commonly owned and co-pending application Ser. No. 09/356,384, filed Jul. 16, 1999, titled: xe2x80x9cBaseband Signal Converter Device for a Wideband Impulse Radio Receiver,xe2x80x9d which is also incorporated herein by reference.
The output of the correlator 710 (FIG. 1a), also called a baseband signal 712, is coupled to a subcarrier demodulator 732, which demodulates the subcarrier information signal from the subcarrier. The purpose of the optional subcarrier process, when used, is to move the information signal away from DC (zero frequency) to improve immunity to low frequency noise and offsets. The output of the subcarrier demodulator 732 is then filtered or integrated in a pulse summation stage 734. The pulse summation stage produces an output representative of the sum of a number of pulse signals comprising a single data bit. The output of the pulse summation stage 734 is then compared with a nominal zero (or reference) signal output in a detector stage 738 to determine an output signal 739 representing an estimate of the original information signal 616.
The baseband signal 712 is also input to a low pass filter 742 (also referred to as lock loop filter 742). A control loop comprising the low pass filter 742, time base 718, precision timing generator 714, template generator 728, and correlator 710 is used to generate a filtered error signal 744. The filtered error signal 744 provides adjustments to the adjustable time base 718 to time position the periodic timing signal 726 in relation to the position of the received signal 708.
In a transceiver embodiment, substantial economy can be achieved by sharing part or all of several of the functions of the transmitter 602 and receiver 702. Some of these include the time base 718, precision timing generator 714, code source 722, antenna 704, and the like.
As described with reference to FIG. 1a, the circuit or device in an impulse radio receiver that converts the received impulses into a baseband signal is sometimes referred to as a cross-correlator or sampler. The baseband signal converter of an impulse radio receiver typically integrates one or more pulses to recover the baseband signal that contains the transmitted information. One embodiment of a cross-correlator device usable in an impulse radio receiver is described in U.S. Pat. No. 5,677,927, issued Oct. 14, 1997, and assigned to Time Domain Corporation. The disclosure of the ""927 Patent is incorporated in this specification by reference.
As noted above, prior art wideband impulse radio systems typically use a binary (or scalar) modulation/demodulation scheme in which the transmitter emits a wideband pulse whose time position is varied in response to two separate components: an information component and a pseudo-random (PN) code component. The PN code component functions, in a general sense, as a CDMA (Code Division Multiple Access) channel sequence. Binary time shift modulation can be implemented by shifting the coded time position by an additional amount (that is, in addition to the PN code dither) in response to the information signal.
In a typical system, the information component provides a modulation on the order of xc2xc cycle of the pulse waveform. At the receiver, correlation occurs to determine if the transmitted pulse is early or late, with the result of this determination defining whether a digital xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d has been transmitted. Multiple pulses can be combined to improve the signal-to-noise ratio of the received signal, but this will cause a reduction in the rate at which data can be transmitted and received.
Using prior art modulation schemes in ultra-wide band impulse radio systems, the practical limitation on the maximum data transmission rate is one bit per pulse. In addition, there are limitations on the pulse rates that can be practically achieved. This suggests the need for methods of increasing the data rate without increasing the pulse rate or adversely degrading the bit error performance.
A typical prior art m-ary modulation scheme used in narrow band radio systems might use two correlators receiving the same signal shifted 90 degrees in phase. This yields a correlation function as shown in FIG. 2 where the I and Q channels represent the outputs of the two correlators such that the Q channel is shifted 90 degrees from the I channel. Four potential modulation states M1-M4 are shown as an example of the case where M is four. When these two functions are plotted with respect to one another, the plot of FIG. 3 results. The four modulation states M1-M4 are also shown in FIG. 3.
Again referring to prior art m-ary modulation, FIG. 4 is a plot of the two correlation functions when the sampling sine wave is 180 degrees. FIG. 5 shows these two functions plotted versus one another. It can be seen that when the modulation is separated by 180 degrees, the I correlator generates an inverted version of the output of the Q correlator. Thus, only two end points may be observed and, accordingly, fewer modulation states can be achieved using those end points and without resorting to amplitude variations.
What is needed, then, is a modulation method and system for use in wideband impulse radio communications that can provide data transmission rates in excess of one bit per pulse with acceptable bit error rate performance in the presence of noise.
In accordance with one object of the invention, a method and system have been developed for time modulating and demodulating wideband impulse radio signals to provide a data transmission rate in excess of one bit per pulse, while maintaining an acceptable bit error rate in the presence of noise. This improved data rate is achieved by a novel modulation scheme within the information component of the wideband impulse radio signal. Although time position modulation of the pulses is still used, the modulator portion of the impulse radio transmitter imposes multiple modulation states on each transmitted pulse. Multiple correlators (or baseband signal converters) in the impulse radio receiver can distinguish each of the multiple states so that each pulse can now communicate two or more bits of data.
In one embodiment of the novel modulation method of this invention, each transmitted pulse is modulated into one of four different time positions spanning a cycle of the pulse interval. Accordingly, four demodulation vector states, or quadrants, are determined relative to the correlator response function. These four transmitted time positions and demodulation vector quadrants correspond to two data bits of information: (0,0), (0,1), (1,0), and (1,1). In accordance with calling two state modulation binary, this method may be termed quaternary or more generally m-ary, where m can be any integer two and above, subject to system performance considerations. The value of m may be selected to be equal to an integer power of two for convenience in mapping to binary data. Thus where m=2n, n data bits may be represented. In one embodiment designed to maximize the data rate, each pulse constitutes a xe2x80x9csymbolxe2x80x9d, with data transmitted at n bits per symbol and one pulse per symbol. In another embodiment designed to improve the signal-to-noise ratio and/or to provide error correction, a sequence of multiple pulses, each modulated into the same relative time position or modulation position by the information signal, can be combined to form a single n bit data symbol by summing pulses to form a symbol. A variation of this embodiment requires the transmission and summation of a sequence of multiple pulses, each modulated into different time positions or modulation positions, to form a single n bit data symbol.
In the receiver, two correlators are used to demodulate the information component from each pulse, i.e., to estimate the values of the data bits being transmitted with each pulse. The first correlator observes each pulse at a reference point (the xe2x80x9cIxe2x80x9d channel) according to a reference clock while a second correlator observes each pulse at a time offset point (the xe2x80x9cJxe2x80x9d channel) from the reference clock. The second correlator is responsive to a decode timing signal used to trigger operation of the first correlator, but delayed by an amount of time, typically one-quarter to one-half of the RF pulse period. Each correlator typically includes an integrator and may include a sample and hold or track and hold circuit to overcome droop and other imperfections in practical high speed integrator circuits. These circuits are collectively referred to as the xe2x80x9ccorrelatorxe2x80x9d for the remainder of this document.
The outputs from the correlators are supplied to a lock loop circuit that is responsible for acquiring and locking, i.e. synchronizing, the receiver time base with the transmitter time base. The lock loop circuit may include a thresholding circuit for acquiring the signal generated by the transmitter.
In one embodiment, the output of the I correlator is compared in a first comparator to a first reference voltage and the output of the J correlator is compared in a second comparator to a second reference voltage in order to estimate the values of the data bits being transmitted with each pulse. The reference voltages may be equal and may be adjusted with respect to each other in order to improve identification of the data bits transmitted with each pulse. In a second embodiment, the outputs of both correlators are compared in a comparator to a long term average output from each correlator, with the outputs of the comparators to achieve an estimate of the value of the two data bits associated with each pulse.
In a third embodiment, the output of the I correlator is summed with the output of the J correlator and the output of the I correlator is subtracted from the output of the J correlator. The summed value (I+J) is then compared in a first comparator to a first reference voltage and the subtracted value (Ixe2x88x92J) is compared in a second comparator to a second reference voltage in order to estimate the values of the data bits being transmitted with each pulse. As before, the reference voltages may be equal and may be adjusted with respect to each other to improve identification of the data bits transmitted with each pulse.
In still another receiver embodiment, the analog I and J channel outputs from the first and second correlators are digitized and sent to a processor. The processor contains vector demodulation algorithms to estimate the value of the n data bits associated with each pulse. In one embodiment of a quaternary demodulation algorithm, the processor compares the digitized values of the I and J channel outputs for each pulse to a table of stored I and J channel value pairs. Four specified ranges of I and J channel value pairs are grouped in memory, with each group corresponding to expected scatter points associated with one of the four modulation states. Based on the results of this comparison, the processor provides an output representing the two data bits associated with that pulse: (0,0), (0,1), (1,0), or (1,1). The correlators and processor are then reset, so that processing of the next pulse can proceed.
If the wave shapes of the transmitted RF pulses are expected to be changed by environmental conditions near the receiver, the processor can implement additional algorithms to make adjustments in real time to maintain an acceptable degree of separation between the modulation states. In one embodiment, the processor can separately monitor the average values of the I and J channel outputs to determine if an acceptable amount of separation exists between modulation states. Alternatively, or in addition, the processor can store a sequence of scatter points generated by the I and J channel outputs and periodically compare them to an idealized plot of the expected scatter points in each modulation state or quadrant, as stored in memory. In either case, the processor can compensate for error by providing a signal to the J channel timing delay circuit that will vary the amount of delay between triggering of the I and J channel correlators, until separation among the four modulation states is optimized.
In a further embodiment, where transceivers are employed, the two transceivers can exchange signal performance information to determine the optimal modulation states for a given propagation path. This improvement is potentially significant since the pulse shape can be modified by objects or reflectors in the propagation path.